Gas turbines must be driven to rotate at a starting speed by auxiliary means prior to fuel injection and ignition and self-sustained operation. In the past, for example, gear box systems driven by auxiliary electric or compressed air motors have been used to rotate the turbine to starting speed. "Air" impingement starting systems have also been used with small turbines and operated by directing a stream of gas, typically air, onto the turbine or compressor wheel to cause rotation of the main rotor. These prior art systems are complex and difficult to implement.
Electrical power may be generated by using a gas turbine to drive an alternator. The alternator may be driven by a free turbine which is coupled to the rotor of the alternator or through a gear box. In these conventional systems, the speed of the turbine must be precisely controlled to maintain the desired frequency and voltage of the generated alternating current output power. Hence, both the output voltage and frequency are fixed for a given system.